1. Field of the Invention
The present invention relates to a transmitter for multi-carrier transmission and a multi-carrier transmitting method.
2. Description of the Related Art
In recent years, radio transmission systems using multi-carrier transmission have been intensively studied.
For example, “IEEE802.11a”, which is a standard of wireless LAN, defines a radio transmission system to be performed using OFDM (Orthogonal Frequency Division Multiplexing) which is a simple type of multi-carrier transmission.
Radio access systems such as MC/DS-CDMA (Multi-carrier/Direct sequence-CDMA) and MC-CDMA (Multi-carrier-CDMA) are proposed as radio transmission systems combining the multi-carrier transmission with CDMA (Code Division Multiple Access).
As shown in FIG. 1, spreading code multiplying units 131 to 13n spread data symbols using spreading codes, create DS-CDMA signals by arranging (mapping) the spread data symbols (chips) in the time-base, and transmit a plurality of DS-CDMA signals over sub-carriers in parallel, in the MC/DS-CDMA.
As shown in FIG. 2, spreading code multiplying units 131 to 13n spread data symbols using spreading codes, arranging (mapping) the spread data symbols (chips) in a plurality of sub-carriers, and transmit the arranged data symbols (MC-CDMA signals) over the sub-carriers in parallel, in the MC-CDMA.
In this way, the reason why radio transmission systems using multi-carrier transmission have been intensively studied is that the multi-carrier transmission has a tolerance to multi-path propagation which is caused by signals transmitting from a transmitter to a receiver via a plurality of propagation paths.
When multi-path propagation occurs, there is a problem in that a delayed signal reaching the receiver can interfere with an earlier received signal.
However, in the multi-carrier transmission, a symbol length becomes longer, and a delay between the early signal (for example, a symbol over the path #1) and the delayed signal (for example, a symbol over the path #L) is smaller than the symbol length as shown FIG. 3. Therefore the influence of interference caused by the multi-path propagation is reduced in the multi-carrier transmission.
Information to be transmitted is overlaid on the amplitude or the phase of the signals transmitted from the transmitter. Therefore the receiver needs to remove any change in amplitude and/or phase which occurred in the propagation paths, and reconstitute the information which is overlaid on the amplitude or the phase.
The method for transmitting symbol patterns (pilot symbol patterns), in which the amplitude and phase are known in the transmitter and the receiver, between the transmitter and the receiver, and estimating the change in amplitude and/or phase which occurred in the propagation paths, are known.
Therefore the configuration of the pilot channel which multiplexes the pilot symbol patterns is very important in the multi-carrier transmission.
Regarding the configuration of the pilot channel in the multi-carrier transmission, the configuration of the pilot channel in the OFDM is disclosed in JP2001-203665, the configuration of the pilot channel in the MC/DS-CDMA is disclosed in JP2001-244913, and the configuration of the pilot channel in the MC/CDMA is disclosed in JP2001-197037.
The above documents disclose the configuration for multiplexing pilot symbol patterns in at least one direction of the time base, the frequencies (sub-carriers) or codes.
The method for spreading data symbols in the direction of the time base and the sub-carriers in two-dimensions, that is to say, the method for spreading data symbols over a plurality of symbol durations in a plurality of sub-carriers is disclosed in “OFDM-CDMA using a method for spreading in the time and frequency domain simultaneously (the technical report RCS-200-3 published by the Institute of Electronics, Information and Communication Engineers)”.
However, the conventional configuration of the pilot channel has a problem in that the influence of the interference between the pilot channels multiplexed on the radio frames is not considered.
The conventional method for spreading the data symbols in at least one direction of the time base or the sub-carriers has a problem in that the merit of the signal transmission characteristics in each method greatly depends on the condition of the propagation paths.